At present, the television broadcast of a voice-multiple system is practised and the quality of voice or sound in the television broadcast is remarkably improved. In association therewith, it is required that the sound field thereof are be improved. Further, in view of remote control operation of a television receiver, it is also desired that the tone control can be remotely controlled with a DC voltage. Moreover, when the circuitry of the television receiver is made as an integrated circuit, be it is necessary that the tone control circuit constructed as an integrated circuit.
In consideration of the above points, the inventors have already proposed a tone control circuit which is different from the prior art attenuation type CR system and the feedback NF system.
FIG. 1 shows the above proposed tone control circuit in which an input signal from an input terminal 10 is supplied to an adder 20. The input signal from the input terminal 10 is also fed to a boost signal line consisting of a filter 31, an amplifier 32 and a gain control circuit 33 and the output signal therefrom is fed to an add input of the adder 20. The input signal from the input terminal 10 is supplied further to a cut signal line consisting of a filter 41, an amplifier 42 and a gain control circuit 43 and the output signal therefrom is supplied to a subtract input of the adder 20. At the adder 20, the signal transmitted through the boost signal line is added to the input signal and the signal transmitted through the cut signal line is subtracted from the input signal. Thus, an output signal is derived from the adder 20 and is supplied to an output terminal 60. A voltage V.sub.C derived from a volume control 50 is applied to the gain control circuits 33 and 43 as the gain control signals.
With the above circuit, if the filters 31 and 41 are each made or used as a high pass filter, the boost and cut in a high frequency band are obtained, while if the filters 31 and 41 are each made as a low pass filter, the boost and cut in a low frequency band are obtained.
That is, when the control voltage V.sub.C is in the range from a center value V.sub.O to zero, the gain control circuit 33 is set such that its gain is zero. On the contrary, the gain control circuit 43 is so set that when the control voltage V.sub.C is in the range from the center value V.sub.O to the maximum value 2V.sub.O, its gain becomes zero. Accordingly, when the control voltage V.sub.C is at the center value V.sub.O, the output signals from the gain control circuits 33 and 43 are both zero. Thus, at the output terminal 60 the input signal unchanged will be obtained unchanged and, the frequency characteristic at this time will be flat.
When the control voltage V.sub.C is changed from the center value V.sub.O to the maximum value 2V.sub.O, it is set so that the gain of the gain control circuit 33 increases and becomes 1 at the maximum value 2V.sub.O. Accordingly, at this state, if the gain of the amplifier 32 is taken as A.sub.1, as shown in FIG. 2 the gain of the boost signal line becomes A.sub.1. Therefore, the boost becomes a high boost of A.sub.1 +1 where of course A.sub.1 &gt;0.
On the contrary, when the control voltage V.sub.C is changed from the center value V.sub.O to zero, it is set so that the gain of the gain control circuit 43 increases and becomes 1 at zero. Accordingly, at this state, if the gain of the amplifier 42 is taken as A.sub.2, as shown in FIG. 3 the gain of the cut signal line becomes A.sub.2. Therefore, the cut becomes a high cut of 1-A.sub.2 where 0.ltoreq.A.sub.2 .ltoreq.1. Thus, if A.sub.2 =1, the attenuation can be made infinitive.
As to the boost and cut in the low frequency band, if the filters 31 and 41 are each made as a low pass filter, the operation of the circuit becomes entirely similar to that described above.
According to the above circuit, since the control curves can be freely set both in the boost and cut directions, the characteristic which is preferable for acoustic response can be easily obtained. Further, since the control 50 has no relation to the constants of the filters 31 and 41 and is controlled by the control voltage V.sub.C, the circuit is suitable to be remotely controlled.
For simplifying the circuit, it may be desired as shown in FIG. 4 that the filter 31 be used in common to both of the boost and cut lines. When the circuit is made as an integrated circuit, if only one filter 31 is used the number of pins for capacitors to be externally connected is reduced by one and hence the cost of the circuit can be substantially decreased.
However, if the filter is used in common in the cut and boost lines, the boosting frequency and cutting-off frequency do not coincide on the abscissa of frequency f as shown in FIG. 5, because the boost gain and the cut gain are different as described above. This is not desirable for auditory response.
Accordingly, this invention has an object to provide a tone control circuit which can be controlled by a DC voltage, and which is suitable to be formed as an integrated circuit and which has cut-off frequencies which are coincident with each other between the boost and cut modes.